Compact ranges are useful in limited space settings for testing the radar cross section of objects and for measuring the patterns of antennas. Such ranges offer cost savings over conventional outdoor range systems requiring a long, unobstructed space between the illuminating source and the illuminated object. Compact ranges offer enclosed testing where conditions can be better controlled than in an outdoor setting.
In typical applications, the target of a radar is located at a distance of many wavelengths from the radar source, such that the target is illuminated by plane waves; a condition known as the "far field". Similarly, objects illuminated by antennas are typically many wavelengths from the antenna, so as to be in the far field of that antenna. When measuring the radar cross section of targets, or when measuring the patterns of antennas, far field conditions must be provided to the maximum extent possible.
A compact range must produce plane waves throughout the volume occupied by the object under test. This "quiet zone" or "target zone" must be uncontaminated by spurious sources of electromagnetic energy arising in the range itself.
One way to construct a compact range is to use a paraboloidal reflector illuminated by a source at the focus of the reflector. However, diffraction from the real edge of the reflector distorts the otherwise plane waves propagated from the reflector and contaminates the quiet zone. The challenge is to develop a reflector which appears, within the region of the electromagnetic spectrum to be used for testing, to have no diffracting edge.
Existing reflectors have included various geometrics to achieve plane wave transmission and reduced diffraction. United Kingdom patent publication 817,170 discloses a conceptual reflector made up of a pair of surfaces separated by a medium. The purpose of the patented reflector is to reflect incident signals in parallel waves. The patent, however, does not specifically describe the geometry of the reflector not the precise edge configuration of the reflector so diffraction is reduced.
Carl Pistorius used his Ph.D. dissertation at Ohio State University in 1986 entitled, "New Main Reflector, Subreflector, and Dual Chamber Concept For Compact Range Applications," to propose an edge treatment to a paraboloidal reflector for propagating plane waves and reducing diffraction. His edge modification technique, called rolled edge blending, generated a contour by continuing the parabolic curvature along lines drawn radially from the vertex of the paraboloid. He then mixed the contour of the parabola with the contour of an ellipse tangent to the parabola at the rim. He blended these shapes to insure a smooth transition between the parabolic curve and the elliptical curve. The Pistorius edge can be described by the formula: EQU rolled edge=parabola.times.(1-blending-function)+ellipse.times.blending-function
where the blending function is a combination of cosine functions. This proposed shape, however, still had the shortcoming of producing a discontinuities in the radius of curvature. Consequently, stray fields from edge diffraction were not totally eliminated.
Pistorius next proposed making the reflecting area of the paraboloid in the form of a rectangular section. He did not discuss how to apply his proposed edge treatment to this rectangular shape. Additionally, Pistorius had no effective design for the corners of the rectangular section of the paraboloid. These corners generated diffracted fields. They seriously contaminated the quiet zone of the reflector. Yet for many applications, a rectangular shape is attractive.
The deficiencies in existing reflector shapes were the incentives for developing the geometry of the applicants' invention.
An object of the invention is to provide a reflector which produces plane waves for a compact range. A second object is to provide a design for a compact range reflector which reduces electromagnetic contamination by diffraction.
Features of the invented rolled edge paraboloidal reflector are the offset rectangular-aperture with blended, rolled edges and shaped corners which are mathematically continuous in all directions with the paraboloidal surface.
The advantage of the invention is the reflector allows objects to be tested for their radar cross section, or antenna patterns to be measured, in a compact range with results which duplicate the electromagnetic illumination in the operating environment.